Onset of $η$-nuclear binding in a pionless EFT approach

Abstract

$ηNNN$ and $ηNNNN$ bound states are explored in stochastic variational method (SVM) calculations within a pionless effective field theory (EFT) approach at leading order. The theoretical input consists of regulated $NN$ and $NNN$ contact terms, and a regulated energy dependent $ηN$ contact term derived from coupled-channel models of the $N^{\ast}(1535)$ nucleon resonance plus a regulated $ηNN$ contact term. A self consistency procedure is applied to deal with the energy dependence of the $ηN$ subthreshold input, resulting in a weak dependence of the calculated $η$-nuclear binding energies on the EFT regulator. It is found, in terms of the $ηN$ scattering length $a_{ηN}$, that the onset of binding $η\,^3$He requires a minimal value of Re$\,a_{ηN}$ close to 1 fm, yielding then a few MeV $η$ binding in $η\,^4$He. The onset of binding $η\,^4$He requires a lower value of Re$\,a_{ηN}$, but exceeding 0.7 fm.